Blendshape the armature
Put the cursor on the center (Shift C), use the cursor as pivot point. Select the mesh in object mode, turn off the armature modifier, press S to scale around the cursor, move your mouse but don't click: in the left bottom corner you will read the amount of scaling according to the mouse movements, read the value when you're satisfied (let's say it's 0.563), press .563 and Enter. Then select the armature, go to edit mode, activate all the armature layers, press A to select all, then press S .563 Enter. Reactivate the armature modifier.